The present invention relates to a diffusion plate which can be used to capture a real image in an optical system and to a method of manufacturing such a diffusion plate.
Diffusion plates are intended to diffuse visible light over a defined divergence angle in the forward and reverse directions. The phase information of the incident lightwave is then destroyed by the diffusion at individual diffusion centers.
In the past, a semi-transparent plate having a roughened surface was used as a diffusion plate for capturing a real image in an optical system, with the incident radiation being scattered by such a plate to a greater or lesser degree. More recently, conventional diffusion plates are provided with roughened, or periodic surface structures. These diffusion plates suffer from certain drawbacks.
Diffusion plates provided with statistical roughening are produced by etching or mechanical grinding and appear grainy if the incident light enters through a small aperture. Moreover, relatively strong scattering is required to completely destroy the phase relationship between the incident light and the transmitted light. This causes light to be lost for the subsequent optical system.
In plates provided with periodic surface structures, the entrance pupil of the subsequent optical system is not illuminated uniformly and a refracted image appears in the entrance pupil. To obtain optimum performance, however, the entrance pupil must be illuminated uniformly.
It is known, in principle, to produce etched cones or conical holes in glass by means of ion irradiation and subsequent etching in hydrofluoric acid. However, glass plates treated in this manner do not offer any particular advantages, compared to conventional glass plates, for use as diffusion plates since the cone angles are fixed. In common window glass, for example, the etched cones have an aperture angle of about 70.degree..
Further, it is known to treat optical surfaces against reflection by irradiation with heavy ions and subsequent etching. The basic principle here is the removal of reflective properties by a refraction index which changes with the depth of the material. In such an anti-reflection treatment, the incident light should be diffused or reflected as little as possible, and the phase information of the incident light should remain completely unchanged.
According to the present invention, recesses in diffusion plates must always be wider than they are deep to achieve their intended purpose. In contrast thereto, the recesses of a surface treated against reflection are always more deep than wide. In the above-mentioned anti-reflection treatment, the resulting structures must be spaced laterally from one another at distances which are significantly smaller (e.g. 10 times smaller) than the length of a lightwave, so as to diffuse the incident lightwave as little as possible. Moreover, arbitrary depth structuring is sufficient for an anti-reflection treatment, as long as the transition from an average density of .rho.0 to the average density of the anti-reflection treated material occurs only gradually at a depth which corresponds to, or is greater than, the wavelength.
Finally, in an anti-reflection treatment of surfaces by means of irradiation with heavy ions, the nuclear trace density must be at least 10.sup.10 /cm.sup.2. In a dosage range below 10.sup.10 ions/cm.sup.2, small scattering objects (smaller compared to the wavelength of light) would act as diffuse scatterers and would destroy the phase coherence of the incident light. Thus, below an irradiation density of 10.sup.10 ions/cm.sup.2 an undesirable diffusion background appears during an anti-reflection treatment.
In light of the above, such methods for producing non-reflective surfaces by means of ion irradiation and etching do not provide any guidance for the production of ground glass diffusion plates which must serve precisely opposite purposes. The process steps employed in an anti-reflection treatment are not only unable to improve the diffusion characteristics of a surface, but also are not permitted to improve such characteristics.